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ISATIN                                      229
Reaction.—Make a small quantity of solution of alizarin in
caustic soda, and pour into a beaker containing a strong solution
of alum. The insoluble aluminium alizarate is precipitated as a
red lake. See Appendix, p. 316.
Isatin from Indigo,  C,H,<       ^C(OH)
\N x/
Erdmann J. prakt. CJicm^ 1841, 24, ii; Knop, Jahresb.
1865, 580.
100 grins, indigo (in fine powder).                                                              <
SOG.C. cone, nitric acid diluted with 10 c.c. water.
Mix ii|) the indigo into a paste with 300 c.c. of boiling- water
in a large basin. Heat to boiling and remove the flame. Then
add the nitric acid to the hot liquid from a tap-funnel at the rate
of a drop or two a second, so that it is all added in the course                      \
of twenty minutes, and stir well all the time. The mass, which
is at first pasty, froths up, and towards the end becomes thinner.
Moil up for about two minutes, as soon as the acid has all been                      ,. j
added, and then pour out about half the liquid into a second
large basin and add a litre of boiling water to each. Boil up                      !wj
for five minutes, and decant from the floating lumps of tarry                      A.
matter through a large fluted filter paper previously moistened                       •>
with water.    Add another litre of hot water to each basin, boil up,                      L
and filter.     Evaporate the combined red coloured filtrates to                      ft
about i \ litre, and filter again, if necessary, from a further deposit                      ^
of tar.     On cooling, a quantity of red crystals discoloured with                      1« ,
tar will separate.     Filter and  concentrate  the  filtrate.     Re-                      ,^i
dissolve the crystals in the smallest quantity of boiling water,                       |- ;
and let the liquid cool somewhat, so that some  of the tarry                      ,J*
matter may separate ; filter and  evaporate  the  filtrate,  until                       «''•
crystals of isatin nearly cover the surface ; then cool and filter                       l!
off the red crystalline deposit.     A further quantity of crystals                       ',|
may  be  obtained  by  evaporating  the  mother-liquors,  which                       J1
must be frequently filtered from tarry deposit.    The crystals                       f
obtained in this way may be purified by dissolving them in
caustic potash solution, and adding concentrated hydrochloric                        j.
acid  to  the  clear liquid  so  long as a black   precipitate   is                       ^